1. Field of the Invention
This invention relates generally to a wet type electrophotography apparatus using a non-volatile, high-viscosity liquid toner, and more particularly to a wet type electrophotography apparatus in which blush-free, high-quality images can be developed by feeding and applying a developing solution evenly onto a developing roller and applying a film of prewetting solution evenly onto a photoconductive medium, and a residual toner layer on the developing roller after development can be stably recovered.
2. Description of the Related Art
As for electrophotography apparatus in which an electrostatic latent image is formed on a photoconductive medium (photoconductive drum), a toner is caused to adhere to the charged image, and the powder image is then transferred and thermally fixed onto a printing medium, such as paper, the dry type using a powder toner has been widely employed.
The powder toner, however, tends to be scattered and often involves the problem of poor resolution due to its particles having sizes as large as 7.about.10 .mu.m.
In applications requiring high resolution, therefore, a wet type electrophotography apparatus using a liquid toner is usually adopted. The liquid toner is less subject to distortion in toner images and can achieve high resolution because it contains toner particles as small as 1 .mu.m and has a large charging capacity.
In the conventional wet type electrophotography apparatus, a low-viscosity liquid toner obtained by mixing 1.about.2% of toner in an organic solvent has been commonly used as the developing solution. This type of developing solution, however, causes environmental concerns because it contains an organic solvent harmful to human bodies and requires a large amount of toner consumption due to low concentration.
It was against this background that a wet type electrophotography apparatus using a high-viscosity, high-concentration developing solution obtained by dispersing a high-concentration toner in silicone oil, etc. was disclosed in International Disclosure Number "WO95/08792."
The use of this liquid toner eliminates not only the harm to human bodies but also the need for large consumption of developing solution due to high toner concentration.
As disclosed in "International Disclosure Number WO95/08792," the wet type electrophotography apparatus using high-viscosity, high-concentration liquid toner as the developing solution has employed the same construction as with the dry type apparatus using a powder toner, in which a developing solution is applied to a developing roller or belt, which is brought into contact with a photoconductive medium to cause the toner to deposit on the photoconductive medium on which an electrostatic latent image is formed.
That is, as in the dry type where the powder toner is applied to the developing roller using a purely mechanical contact means, the wet type using a non-volatile, high-viscosity, high-concentration liquid toner also employs a purely mechanical contact means in applying the developing solution to the developing roller. The application of the developing solution is accomplished by finishing the surface which makes mechanical contact with the developing roller to a high-precision finish.
That is, development is carried out by applying a liquid toner onto a developing roller 22, causing the developing roller 22 to face a photoconductive medium 10 and applying a voltage to the developing roller 22, as shown in FIG. 25. At this time, an image area and a non-image area (background area) are formed randomly in accordance with an image pattern on the toner layer on the developing roller 22 after development. Rollers 23.about.25 in FIG. 25 are a series of applicator rollers for feeding the developing solution to the developing roller 22. These rollers are driven by drive motors 31 and 33, and gears 32 and 34.
If the image area and the non-image area on the developing roller 22 after development, which have different electrical histories on the toner and different amounts of residual toner, are brought as they are to the contact area at which the developing roller 22 makes contact with the final-stage applicator roller 23, the evenness of the toner layer formed on the developing roller 22 becomes a problem. Even if the toner layer is formed evenly, the image pattern at the previous rotation of the developing roller 22 may appear on the image in synchronism with the period of the roller due to the electrical history experienced by the toner. Such a phenomenon is a trouble often referred to as the development memory. It is necessary therefore to have a reset mechanism for erasing the history left as the result of a development operation.
A problem characteristic of the liquid developing process is that when a prewetting solution is used as a release agent to prevent blushing, the prewetting solution deposited on the developing roller 22 causes the dilution of the developing solution. To cope with this problem, a rubber blade 37 has heretofore been used to scrape off the residual toner layer on the developing roller 22.
As a means to solve the trouble called the development memory and the dilution of the developing solution, the use of the rubber blade to scrape off the residual toner layer is effective only when the blade is used to wipe off something on a surface of a relatively hard material, as with the automotive windshield wiper. However, when the rubber blade is used to scrape off the residual toner on a developing roller of a relatively soft developing roller, the contact pressure between both cannot be satisfactorily maintained.
The pressure between the blade and the roller can be considerably increased by forcing the edge of the blade onto the roller in the "doctor" direction ("biting" direction in which the blade faces the roller at an acute angle). However, but when used with a soft roller with a low hardness, the doctor-blade effect is drastically reduced, leading to an increase in the amount of toner left on the roller.
As disclosed in International Disclosure Number "WO95/08792," when a developing device is constructed using a flexible developing belt, adjustment of the developing belt is extremely difficult, requiring much labor and time. Too tight a developing belt would reduce the flexibility, making the belt too rigid. This makes it impossible to realize the double-layer construction of the liquid toner applied to the developing belt and the prewetting solution layer applied to the photoconductive medium. Too loose a developing belt, on the other hand, would cause a gap between the liquid toner on the developing belt and the prewetting solution layer applied to the photoconductive medium, preventing the movement of the toner. In this way, a developing device using a developing belt would make the adjustment of the developing belt difficult and time-consuming.
When a high-viscosity liquid developing agent is used, the following requirements must be satisfied to form a thin film of the developing agent through the use of a contact roller while maintaining the prewetting solution layer undisturbed. First, the total amount of developing and prewetting agents carried along by the rotation of the photoconductive medium and the developing roller must be passed at a pressure below the contact pressure at the contact area between the developing roller and the photoconductive medium. The lower the hardness of the developing roller the more liquid can be passed. However, the higher the dimensional accuracy of the outside diameter and the wobbling accuracy during rotation of the developing roller, the more stably the pressure on the liquid layer is maintained. In terms of machining, the lower the hardness the more difficult becomes the improvement of machining accuracy.
Second, there must be a prewetting oil layer at all times over the entire region between the developing agent and the surface of the photoconductive medium so as to ensure that the high-viscosity developing agent is kept from directly contacting the surface of the photoconductive medium. If the surface of the high-viscosity toner layer is not a uniform plane, having irregularities, projected parts of the toner layer may pierce through the prewetting solution layer, coming in direct contact with the surface of the photoconductive medium, impairing the releasing effects of the prewetting agent. This may lead to blushing (deposition of unnecessary toner) on the non-image area. The uneven layer on the surface of the developing roller appears on the image area, resulting in a poor image.
In the meantime, there also have been some problems with prewetting treatment.
When a non-volatile, high-viscosity, high-concentration liquid toner is used, prewetting treatment is carried out by applying a prewetting agent, such as silicone oil, to the photoconductive medium prior to the application of the liquid toner to prevent the high-viscosity toner from adhering to non-exposed areas on the electrostatic latent image formed on the photoconductive medium. The prewetting solution layer applied during prewetting treatment helps prevent the toner from adhering to the non-exposed area on the photoconductive medium, as shown in FIG. 12. To ensure stable application of an electrical field and prevent the toner from adhering to the surface of the photoconductive medium as a blush, it is very important to apply the prewetting agent evenly onto the photoconductive medium.
Two methods of applying the prewetting solution on the photoconductive medium are heretofore known; one involving the use of a stationery applicator member, such as felting, and the other involving the use of a roller train comprising a plurality of rollers.
The use of a stationery applicator member involves a sponge block of an almost rectangular cross-sectional shape having a capacity to absorb and hold the prewetting solution, as shown in FIG. 36. The prewetting solution is absorbed and held inside sponge cells based on a capillary action or a gravitational effect, and the sponge is brought into contact with the photoconductive medium 10 in an appropriate manner to apply the prewetting agent to the photoconductive medium 10.
Being of a simple construction, this method can be effective in applying an even coating film if an appropriate material is selected and this method is used for the apparatus operated at relatively low speeds.
The use of a roller train, on the other hand, involves applying the prewetting solution to the photoconductive medium 10 by forming a prewetting agent film on rollers, which is in turn caused to make contact with the photoconductive medium and rotate at the same speed as the photoconductive medium, as shown in FIG. 36. The roller train can be some combinations of rollers made of open-cell sponge, or solid rollers, etc.
With this method, the prewetting agent can be applied on the photoconductive medium in a smooth film if the apparatus is operated at low speeds and a low-viscosity prewetting agent is used due to the self-recovery of the prewetting solution resulting from the effects of gravitation and surface tension. When a high-viscosity prewetting solution is fed to the roller train operated at the same speed, however, it is generally known that a remarkably mottled pattern tends to be formed in the neighborhood of the exit.
As a result, even after prewetting treatment, image noise may be produced because the toner is undesirably deposited on the non-exposed area on the electrostatic latent image on the photoconductive medium, on which the toner is not desired to be deposited.
To describe this problem more specifically, now assume a developing device in which a liquid toner is fed from the toner pool while spreading into a thin film using a plurality of interconnecting applicator rollers to form a toner layer on the developing roller, and depositing the toner that is electrostatically charged by an electrical field formed between the developing roller and the photoconductive medium on the exposed area on the photoconductive medium. At the contact portion of the developing roller 22 and the last-stage applicator roller 23, the toner layer receives a compressive force from both rollers, as shown in FIG. 11. As the toner layer approaches the exit of the nip portion, the toner is relieved of the compressive force, and instead receives a tensile force due to the viscosity of the toner itself and its adherence to the roller surface. Thus, the toner layer is separated into two layers on the developing roller 22 and the applicator roller 23, accompanying the toner in the surrounding area. This results in fine streaks (irregularities) left on the developing roller.
As a result, as the developing roller 22 makes contact with the photoconductive medium 10, projected parts of the fine streaks of the toner on the developing roller 22 pierce the prewetting solution layer on the photoconductive medium 10, reaching the surface of the photoconductive medium 10, as shown in FIG. 10. This causes noise in the non-exposed area. This uneven toner layer appears on the exposed area, leading to a poor image.
In this way, the prior art tends to produce image noises because the toner may be deposited on the non-exposed area of the static latent image on the photoconductive medium on which the toner is originally not to be deposited. The prior art also tends to be involved with blushing or blurred edges.
Observation results of dots or fine-line edges on an image formed and developed with a liquid toner with the toner particle content of 20 wt. % and a viscosity of 600 mPa.multidot.S revealed that there were blurred edges on the image. This was attributable to that the toner tends to run due to the low cohesive strength of the toner layer on the photoconductive medium 10, and that when the toner layer is dispersed at high concentrations on the developing roller 22, the toner layer at the edge parts are not clearly separated in both the image and non-image areas. When a toner having the toner particle content of 10% and the same thickness is formed on the developing roller 22 and developed, dots or fine-line edges on the image become very sharp due to the good migration properties of the toner, whereas the image density lowers due to the insufficient amount of toner particles. Increasing the thickness of the toner layer by the amount of lowered image density, however, would increase the tendency to disturb the static latent image formed on the photoconductive medium.
As a means for solving this problem, the photoconductive medium can be exposed only after the prewetting solution has been applied, as disclosed in International Disclosure Number "WO95/08792." This method, however, poses another problem of disturbed images due to fluctuations in the thickness of the prewetting solution layer.
Furthermore, although a cam mechanism for causing the porous sponge to make contact with, or detach from, the photoconductive medium is disclosed in International Disclosure Number "WO95/08792," on what occasion the cam mechanism should be operated has not been studied. This may result in an excess amount of prewetting agent applied to the photoconductive medium.
In International Disclosure Number "WO95/08792," moreover, no mechanism is proposed for adjusting the thickness of the prewetting solution layer applied to the photoconductive medium. As a result, the thickness of the prewetting solution layer applied to the photoconductive medium is invariably kept constant, regardless of the types of liquid toners, even when color images are processed.
Either of the methods involving the use of a stationary applicator, made of felting, for example, or the method using a roller train combining a plurality of rollers to apply a prewetting agent to the photoconductive medium, as described above, is less effective in smoothing the surface of the photoconductive medium at higher process speeds. As the viscosity of the prewetting solution increases, this tendency becomes pronounced. Thus, the prewetting solution tends to be fed in mottled patterns to the developing point, producing uneven image density or blushing.
The mottled patterns of the prewetting agent caused by rotating the rollers in the same direction (as viewed in the nip portion) have been examined in several experimental analyses, but have not yet been fully elucidated theoretically. It is generally believed that as the rollers are rotated in the forward direction (one roller is rotated clockwise and the other counterclockwise), the prewetting solution tends to be sheared and separated by both rollers at the exit of the rollers, producing a raindrop-like pattern called "rivulet." The rivulet pattern has an interval of 0.1.about.1 mm, and the difference between the peak and the bottom is several times as much as the average thickness of the layer. The rivulet is believed to depend on the construction and conditions of the prewetting system, and the viscoelastic properties (rheology) of the prewetting solution.